Generally, crude petroleum contains various salts such as NaCl, MgCl.sub.2, CaCl.sub.2 and the like. Such salts in the crude petroleum should be removed in a desalter of an oil refinery process. In order to remove the salts in crude oil, a lot of degassed water is employed in the desalter. In other words, the degassed water removes the salts from the crude petroleum in the desalter, and discharges from the desalter. The water discharging from the desalter is called as effluent water. The effluent water still contains oil components, phenols, gas components such as H.sub.2 S and NH.sub.3, as well as salts. Accordingly, the effluent water shows a high level of COD. In order to discharge the effluent water to the sea, the effluent water should be treated. In general, the effluent water should be sent to a wastewater disposal plant for a biochemical treatment. As the amount of the effluent water coming out from a petroleum refinery process is about 30 to 60% of the total wastewater from all of the processes, the treatment of effluent water requires a large scale of treatment equipment and costs a lot.
One of the systems for treating effluent water has been developed by Nippon Oil Eng. Construction Co., and the system comprises a slop oil separator and a whole straight run ("WSR") separator (hereinafter, "NECC system"). It is possible to remove oil components in the effluent water by means of the NECC system, whereas COD components or gas components such as H.sub.2 S and NH.sub.3 in the effluent water may not be removed perfectly in the NECC system.
Accordingly, the present inventors have developed a system for treating effluent water coming out from a desalter so as to remove all the contaminating components, to lower the COD lcvel of the effluent water, and finally to discharge the effluent water to the sea directly or to recycle the effluent water for other purposes.